Conventionally, various binding methods have been proposed and used for binding papers, such as by means of staplers, cords, yarns or wires.
However, the binding by staplers has been found inconvenient in that it is not suitable for binding a stack of papers of a large height, because of the limited size and shape of the staplers. In addition, the staplers are apt to get rusty, and the papers are likely to be damaged.
In the binding method using yarns or cords, a considerably long time is required for the binding work even by skilled hands. At the same time, the papers are likely to be torn at the binding bores, causing a fear of dropping and losing of documents.
The binding by means of wires has been unacceptable in that the wires are apt to get rusty, and the papers are likely to be spoiled and damaged, as is the case of the staplers.
The binding of papers on a large scale can be performed by a sewing press. However, this method requires highly skilled hands and, therefore, can be adopted only in binderies.
To overcome these disadvantages, a method has been proposed in the specification of U.S. Pat. No. 3,452,376 which employs binding cord made of a plastic and having a retaining enlarged portion. This method consists in preparing the binding cord, forming a binding bore through the papers, inserting the narrower end of the cord until the enlarged retaining portion comes in contact with the outermost paper, adjusting the length of the cord emerging from the filing bore and melting the cord at its portion emerging from the bore, by means of a hot press, to form another retaining enlarged portion, thereby to bind the papers. This method has advantages in that the file can have a good appearance. In addition, thanks to the flexible nature of the plastic binding cords, the bound papers can be opened widely, so that one can read letters located near the inner ends of the file which can hardly be read in the papers bound by the conventional binding methods. However, this improved binding method also has drawbacks in that it requires troublesome steps of preparing the binding cords having an enlarged portion and then inserting them into the binding bores formed in the papers, which hinders the continuous binding work.